Hi all

Can anybody tell me why when the car is parked at normal ride height the back and front drops after a couple of hours at roughly the same rate, but if i park up and lift the car to maximum height and leave it the front will drop in an hour and a half ish but the back will stay at full height for about a week???????
Just curious really

D

My BX does this, as did my GSA, and the CX did this too each of them after leaving it on high.

I think the reason is that at normal height the cylinders and spheres use relatively little LHM volume to hold at normal and the spheres front and rear are not 'very' compressed - say just under half the volume of the gas at the front pair and much less at the rear - say only a quarter reduced in volume. But when the suspension is put on high, the lever moved forces the 2 corrector valves to their open position overriding against the automatic rods from the anti roll bars. So the car pumps up and hits the top bumpstops. When it's at high the correctors remain fully open and so the accumulator keeps pumping in more LHM pressure, compressing the gas in the spheres beyond their normal height volume until the pressure of the cylinders and sphere gas is the same as the regulator cut out and so it cuts out at about 175 bar.

Unlike at normal height where the car rises from low with pressure in the cylinders of E.g 100 bar at the front and less at the rear say, 80bar, then as it reaches normal height and the correctors close auto style, the accumulator pressure is disconnected and so the pressure builds up in the supply and accumulator to cut out leaving the lower pressures in the cylinders and spheres) and hence less volume of LHM to leak.

It means that given the same rate of leakage in the cylinders, for both heights, the rear suspension has much more LHM volume in its cylinders than compared to the front at normal height (where there's a greater difference between the F-R spheres' gas compressed volumes), also, with rear spheres that have a lower gas pressure than the fronts anyway it means that with the same LHM pressure F-R cylinders at high setting, the rear spheres' gas is compressed much smaller to reach the 175 or so bar of regulator cut out and so takes a larger volume of LHM to reach this, add this to the fact that the front has much more weight and it promotes this effect.

So simply put, the rear has more LHM at the same high pressure to start with and this takes longer to leak and that the front is heavier.

I did find one exception to this in one Citroen! my Dad's old Xantia, a Mk 1 'sinking' type. When I left that on high over night the front was on the stops and the rear had sunk half way and was sitting at about normal height - even when brand new, Mk 1 Xantias seemed relatively fast sinkers until they brought in the anti sink valves.

I think for the rears the reason is mych simpler, there is almost no wear in that section of the cylider bore, so almost no leakage! (have found my car leaks a LOT less if parked on high!)

Stewart

Yes! that too! didn't think of that Zaphod, if nothing else, it'll be for both reasons

Thanks Gents, that makes sense. The back of the car is still at maximum height even now.

D

The same principle seems to apply to the height correctors. One of mine drops rapidly after stopping, unless you put it on full height, it then stays, unless you touch the lever on the height corrector when it immediatly starts dropping with a hissing noise.

Peter.N.